The present invention relates to a process for working up the wash solution obtained in the washing of SO.sub.2 -containing off-gases, which can also contain SO.sub.3, using ammonia.
It is known to wash SO.sub.2 -containing gases, e.g., the end gas from a plant for the production of sulfuric acid by the contact process, or flue gas from the combustion of sulfur containing fuels, by adding ammonia or an aqueous ammonium hydroxide solution to the wash solution. The ammonium hydroxide solution can contain generally 10-30 wt.% NH.sub.4 OH. The wash solution contains primarily ammonium sulfite and ammonium bisulfite. The washing process can be conducted in a variety of ways, e.g. by contacting the SO.sub.2 -containing gas countercurrently with an aqueous ammonium sulfites-containing spray, generally but not necessarily at a temperature of 40.degree. to 60.degree.C and a pressure of about one atmosphere.
In so far as the off-gas also contains sulfur trioxide, or an oxidation potential for oxidation of sulfite to sulfate exists, the wash solution can also contain ammonium sulfate.
U.S. Pat. No. 3,321,275 describes a process for the disposal of this wash solution in an environmentally harmless manner. Hence it is known to acidify the solution by adding ammonium hydrogen sulfate, whereby SO.sub.2 is liberated, to evaporate the solution for recovery of the ammonium sulfate from the solution, and, in stages, to split it into ammonia and SO.sub.2. This process indeed permits the recovery of the ammonia required for the off-gas washing; but it requires a considerable capital expenditure for the necessary multi-stage plant.
U.S. Pat. No. 3,359,069 describes a process for the production of sulfur dioxide-containing gases by the thermal decomposition of waste sulfur acids and their ammonium salts in the presence of oxygen. These waste sulfur acids contain, for example, 34 wt.% water while solutions coming from washing off-gases contain about 55 wt.% water. The thermal decomposition of such wash solutions results in split gases which are too poor in SO.sub.2 for an economic production of sulfuric acid. Combustion of sulfur or hydrogen sulfide in order to increase the SO.sub.2 content of the spent gas would result an unacceptable compulsory production of sulfuric acid.
U.S. Pat. No. 3,383,170 describes a process for recovering ammonia in yields of at least 95% from feeds containing ammonium sulfate and/or ammonium bisulfate by heating to temperatures of 200.degree.-450.degree.C. NH.sub.3 is produced from the decomposition of ammonium sulfate, whereafter ammonium bisulfate is reduced to the bisulfite, which reacts to form SO.sub.2 and ammonium sulfate; the process is controlled so that the amounts of NH.sub.3 and SO.sub.2 correspond to the initial NH.sub.4 and SO.sub.4 content of the feed. The price of ammonia decides whether combustion of ammonium salts of sulfur acids into N.sub.2, H.sub.2 O and SO.sub.2 (U.S. Pat. No. 3,359,069) is more economical than recovery of NH.sub.3 and SO.sub.2 (U.S. Pat. Nos. 3,321,275 and 3,383,170). The present U.S. patent application describes a technical and economical improvement of the process (U.S. Pat. No. 3,359,069) applied for solutions containing ammonium salts of sulfur acids obtained in the SO.sub.2 -containing off-gases.